In recent years, with the spread of mobile electronic devices and high performance requirement thereof, a secondary battery having a high energy density has been desired. As this kind of battery, a lithium ion secondary battery that uses an organic electrolyte (non-aqueous electrolyte) has been under attention. Since this lithium ion secondary battery can generate an average voltage of about 3.7 V, which is about 3 times a voltage of an alkali secondary battery that is a conventional secondary battery, it can achieve a high energy density. However, unlike the alkali secondary battery, the lithium ion secondary battery can not use an aqueous electrolyte, and therefore a non-aqueous electrolyte having sufficient oxidation-reduction resistance is used. Since the non-aqueous electrolyte is inflammable and has danger of ignition or the like, a full attention is paid to safety in the use thereof. Several cases that are exposed to a danger of ignition or the like can be considered, and particularly overcharge is dangerous.
In order to prevent the overcharge, in a conventional non-aqueous secondary battery, a constant voltage and constant current charge is performed, and a precise IC (protection circuit) is provided to the battery. The cost of the protection circuit is large, which makes the cost of the non-aqueous secondary battery high.
In case of preventing the overcharge by the protection circuit, the protection circuit is naturally assumed not to work well in some times, and it is difficult to say essentially safe. In a conventional non-aqueous secondary battery, for the purpose of safely destroying the overcharged battery due to breakdown of the protection circuit during the overcharge, attempts have been made to develop equipment of a safety valve•PTC element, a lithium ion secondary battery separator having a thermal fuse function and the like. However, even when such equipment and function as described above are adopted, the safety of the battery during the overcharge is not surely guaranteed depending on the condition of the overcharge. Actually the fire accident of the non-aqueous secondary battery occurs at the present time.
As a lithium ion secondary battery separator, film-like porous films made of polyolefin such as polyethylene are often used. The porous film has a thermal fuse function (shut-down function) such that when an inside temperature of the battery becomes about 130° C., the porous film is melted and fills micro-pores, thereby preventing a lithium ion from migrating, and shutting off the current. However, when the temperature further rises for any reasons, there is suggested a possibility that the polyolefin itself is melted and short-circuited, and thermal runaway is caused. For this reason, at the present time, a heat-resistant separator that is neither melted nor shrunk even under a temperature of about 200° C. has been developed.
As the heat-resistant separator, there are a non-woven fabric made of a polyester fiber and a non-woven fabric in which an aramid fiber that is a heat-resistant fiber is blended with the polyester fiber. However, since these fabrics have large pore diameters and cause an internal short circuit, they are not practical (see Patent Documents 1 to 3, for example). On the other hand, there have been reported examples in which a lithium ion secondary battery separator is formed by applying various composition treatments on a base material such as a non-woven fabric and a woven fabric for a lithium ion secondary battery separator. For example, there have been reported an example in which a composite is formed by laminating a film-like porous polyolefin film on a base material for a lithium ion secondary battery separator, which is composed of a non-woven fabric made of a polyester fiber, and an example in which heat resistance is imparted to the battery by performing a composition treatment such as incorporation of filler particles, surface coating of a resin or the like to a base material such as a non-woven fabric and a woven fabric for a lithium ion secondary battery separator (see Patent Documents 4 to 6, for example). However, since the non-woven fabric that is used as the base material for a lithium ion secondary battery separator has large pore sizes and low surface smoothness, there were quality problems such that surface irregularity is large when fabricated by surface coating, and fabricating materials such as filler particles and the resin are likely to fall off.
In order to improve the denseness of the base material for a lithium ion secondary battery separator, there has been proposed base materials in which an average fiber diameter of a synthetic short fiber that forms the base material for a lithium ion secondary battery separator is made small and a fiber having a specified fiber diameter and fiber length is contained (see, Patent Documents 7 and 8, for example). Patent Documents 7 and 8 disclose a process for producing a base material for a lithium ion secondary battery separator by a wet method and they mention the dispersibility of the fiber. However in these Patent Documents, neither other producing processes are detailed nor studies are not made on a relationship with quality and handling of the base material for a lithium ion secondary battery separator and the lithium ion secondary battery separator. Further, there have been reported examples in which as a fiber that forms a base material for a lithium ion secondary battery separator, a fibrillated fiber is blended to further improve the denseness of the base material (see Patent Documents 9 to 11, for example). However, also in Patent Documents 9 to 11, detailed studies have not been made on the process for producing the base material for a lithium ion secondary battery separator, and a relationship with quality and handling of the base material for a lithium ion secondary battery separator.